The invention is concerned with a pre-mixing type burner having a multiplicity of slit-like flame ports which are arranged in juxtaposed relationship with each other and at a right angle to the walls of the combustion chamber with one end of each flame port being in contact with the wall and, more particularly, to a pre-mixing type burner having a comparatively large combustion chamber load of the order of 10.sup.7 Kcal/h.multidot.m.sup.3.
The conventional burner of the type stated above will be described hereinafter with specific reference to FIGS. 1 to 3. The description will be made, by way of example, on an assumption that the vapor of a liquid fuel (kerosene) is used as the fuel and that the burning flame is directed downward.
The reference numeral 1 denotes a burner element disposed linearly. The burner element 1 is provided at its inner central portion with a secondary air passage 2. A multiplicity of secondary air ports 3 in the form of parallel slits are formed in the projected part of the burner element 1. The reference numeral 4 denotes a flame port section provided at both sides of the secondary air passage, and provided with a multiplicity of slit-like flame ports 5. The flame port section 4 has a greater thickness at its central portion than at its outer portions. The burner element 1 is made from an extruded aluminum material. The secondary air ports 3 and flame ports 5 are formed by slotting the end of the extruded blank material by means of a slotting milling machine or the like. The flame ports 5 open also in the burner supporting portion 6 formed at the end of the flame port section 4. The secondary air ports 3 and the flame ports 5 extend at a right angle to the wall of the combustion chamber.
The reference numeral 10 denotes an evaporator for liquid fuel which is integrally attached by welding or the like measure to the burner element 1. The evaporator 10 is provided with an electric heater 11. A liquid fuel and primary air flow into the passage 12. An outlet port for pre-mixed admixture gas is designated by reference numeral 13.
Left and right combustion chamber walls 15, formed of die-cast aluminum and arranged in pair, cooperate with each other to define therebetween a combustion chamber 16 and a passage 17 for the pre-mixed gaseous admixture. The aforementioned supporting portion 6 of the burner element 1 is received by a groove 18. Combustion chamber walls 15, 15 are integrally welded to each other. The combustion chamber 16 has a slightly diverging form with a convection type heat exchanger disposed at the diverged end thereof.
The burner element 1 is closed at its one end by a closure plate 20 which is fastened to the burner element 1 by means of screws 22, with a packing 21 interposed therebetween. This assembly is inserted into the pre-mixed gas passage 17 through a packing 23 and is fixed to the combustion chamber wall 15 by means of screws 24. Reference numerals 25 and 26 denote, respectively, a secondary air inlet port and an inlet port for the primary air and liquid fuel.
The reference numeral 27 denotes a closure plate for closing the other end of the burner element 1, the plate being fixed to the latter by means of screws 29 with a packing 28 interposed therebetween. The combustion chamber 16 and the pre-mixed gas passage 17 are closed by a closure plate 30 which is fixed to the combustion chamber wall 15 by means of screws 32.
The reference numeral 33 denotes a cover for covering the portion of the combustion chamber wall 15 defining the pre-mixed gas passage 17. This cover 33 is inserted into grooves 34 and 35 formed in the closure plates 20 and 27. In order to avoid the condensation of the liquid fuel at the time of ignition, the cover 33 is heated by an evaporator 10 through the medium of closure plates 20 and 27.
The reference numeral 36 designates an ignition electrode.
In operation, when the electric power is supplied to the electric heater 11 and the evaporator 10 is heated up to a predetermined temperature, e.g., 250.degree. C., and the supply of air is started by a blower. On the other hand, a liquid fuel is supplied into the passage 12 of the evaporator 10 and is evaporated so as to be mixed with primary air to form a pre-mixed gaseous mixture. The pre-mixed gaseous mixture then flows through the outlet port 13 into the pre-mixed gas passage 17 and is discharged from the latter through the flame ports 5 of the flame port section 4. The discharged pre-mixed gaseous mixture is then ignited by means of the ignition electrode 36 to form a primary flame and is completely burnt with the aid of secondary air introduced through the secondary air port 3.
As the combustion is commenced, the heat of combustion is transmitted through the burner element 1 to the evaporator 10 and then the supply of electric power to the electric heater 11 of the evaporator 10 is stopped.
Since the flame port section 4 has a smaller thickness at its portion closer to the combustion chamber wall 15 than at its portion remote from the same, the flow velocity and flow rate of the pre-mixed gaseous admixture are greater in the region closer to the combustion chamber wall 15 than in the region remote from the same. In other words, the load imposed on the flame ports is greater in the area closer to the combustion chamber wall 15 than in the area remote from the same. Namely, the primary flame formed in the region remote from the combustion chamber wall 15 is a small stable attached flame, while the primary flame formed in the region near the combustion chamber wall 15 is a somewhat lifted flame and the flame adjacent to the combustion chamber wall 15 is formed therealong. Therefore, each slit of the flame port 5 forms a flame shape represented by F.sub.2 in FIG. 4. This flame is a primary flame.
The large flame near to the combustion chamber wall 15 and the stable small flame near to the secondary air port 3 in combination provide a stable burning. This conventional burner, however, suffers the following drawbacks. Namely, when the burner is operated with the heat input of, for example, 17,000 Kcal/h, the ignition is difficult or, alternatively, a resonance sound is generated, for the reason discussed hereinunder.
The aforementioned flame F.sub.2 is formed in the steady state of the burner operation, while, immediately after the ignition at which the temperature is still low, the flame takes a form shown by F.sub.1. As the burner operation proceeds to the steady state, the combustion chamber wall 15 and the burner element 1 are heated by the flame and become effective to heat the pre-mixed gaseous admixture to increase the burning velocity, so that the flame comes to take the form F.sub.2. Namely, the flame surface is shifted at the time of ignition.
It is considered that, since the thickness of the burner supporting portion 6 is smaller than that of the flame port section 4 and since there is a gap 40 between the flame port surface and the groove 18, there is a flow component A of the pre-mixed gas passing from the supporting portion 6 through the gap 40. Therefore, it is believed that the load of the flame adjacent to the combustion chamber wall 15 exceeds the load predetermined on the basis of the thickness of the flame port section 4.
For these reasons, the ignition is somewhat difficult even when the fuel is a town gas if the burning velocity of the gas is of a small value. Furthermore, in a burner which utilizes vapor evaporated from a liquid fuel, the burning velocity is further reduced because the temperature of the pre-mixed gaseous mixture is lowered so as to lower the evaporation temperature for shortening the pre-heating time of the evaporator 10 and the burner element 1 at the starting of the burning. Also, there is a problem that the density of the initial pre-mixed gaseous mixture is low because of the delay of operation of the liquid fuel supply system and/or the delay of evaporation. For these reasons, the ignition is difficult or the flame is undesirably blown out.
It has been found that the above-described problems of the prior art can be overcome by eliminating the flow component A and improving the attachment of the flame to the flame port surface. This can be achieved by, for example, making the open end of the groove 18 contact with the flame port surface to eliminate the gap 40 and to form a flame as shown by F.sub.3.
It is considered that the whole part of the flame F.sub.3 in this state is a perfect laminar-flow attached flame. This flame, however, is liable to be effected by external disturbances and perturbed to generate unpleasant noise (referred to as "resonance noise") as a result of resonance of the frequency of the perturbed combustion with the proper frequency of the whole of the burner.
The improvement in the ignitability and the elimination of the resonance are incompatible with each other.
In addition, if the load on the flame ports is lowered, the flame becomes to be of the form as shown by F.sub.3, the ignitability is rendered poor and resonance noise is generated.